/* 
 * Kodkod -- Copyright (c) 2005-2007, Emina Torlak
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */
package kodkod.ast;

import kodkod.ast.operator.ExprOperator;
import kodkod.ast.visitor.ReturnVisitor;
import kodkod.ast.visitor.VoidVisitor;


/** 
 * A relational {@link kodkod.ast.Expression expression} with two children.
 * 
 * @specfield left: Expression
 * @specfield right: Expression
 * @specfield op: ExprOperator
 * @specfield op.binary()
 * @invariant children = 0->left + 1->right
 * @author Emina Torlak 
 */
public final class BinaryExpression extends Expression {
	private final ExprOperator op;
	private final Expression left;
	private final Expression right;
	private final int arity;

	/**  
	 * Constructs a new binary expression: left op right
	 * 
	 * @effects this.left' = left && this.right' = right && this.op' = op
	 * @throws NullPointerException - left = null || right = null || op = null
	 * @throws IllegalArgumentException - left and right cannot be combined with the specified operator.
	 */
	BinaryExpression(final Expression left, final ExprOperator op, final Expression right) {
		switch(op) { 
		case UNION : case INTERSECTION : case DIFFERENCE : case OVERRIDE :
			this.arity = left.arity();
			if (arity!=right.arity())
				throw new IllegalArgumentException("Incompatible arities: " + left + " and " + right);
			break;
		case JOIN : 
			this.arity = left.arity() + right.arity() - 2;
			if (arity < 1)
				throw new IllegalArgumentException("Incompatible arities: " + left + " and " + right);
			break;
		case PRODUCT : 
			this.arity = left.arity() + right.arity();
			break;
		default : 
			throw new IllegalArgumentException("Not a binary operator: " + op);
		}
		
		this.op = op;
		this.left = left;
		this.right = right;
		
	}

	/**
	 * Returns the arity of this binary expression.
	 * @return this.arity
	 * @see kodkod.ast.Expression#arity()
	 */
	public int arity() {
		return arity;
	}

	/**
	 * Returns this.op.
	 * @return this.op
	 */
	public ExprOperator op() { return op ; }
	/**
	 * Returns the left child of this.
	 * @return this.left
	 */
	public Expression left() {return left;}

	/**
	 * Returns the right child of this.
	 * @return this.right
	 */
	public Expression right() {return right;}

	/**
	 * {@inheritDoc}
	 * @see kodkod.ast.Expression#accept(kodkod.ast.visitor.ReturnVisitor)
	 */
	public <E, F, D, I> E accept(ReturnVisitor<E, F, D, I> visitor) {
		return visitor.visit(this);
	}

	/**
	 * {@inheritDoc}
	 * @see kodkod.ast.Node#accept(kodkod.ast.visitor.VoidVisitor)
	 */
	public void accept(VoidVisitor visitor) {
		visitor.visit(this);
	}

	/**
	 * {@inheritDoc}
	 * @see kodkod.ast.Node#toString()
	 */
	public String toString() {
		return "(" + left + " " + op + " " + right + ")";
	}
}
